E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
By-the-by.
En passant, laterally connected with the main subject. “By-play” is side or secondary play; “By-lanes and streets” are those which branch out of the main thoroughfare. The first “by” means passing from one to another, as in the phrase “Day by day.” Thus “By-the-by” is passing from the main subject to a by or secondary one.
